Search

Boron nitride (BN) has excellent high temperature resistance, thermal conductivity, strong insulation, and wave-transparent properties. Spherical BN is an essential filler in polymer matrix composites due to good particle mobility and high filler content. In this paper, we present a large-scale preparation for BN microspheres with uniform particle size distribution in the range of 1.0–2.8 μm via solvothermal method at low cost and high yield (∼90%), which is two orders of magnitude lower than the diameter of the spherical BN prepared by other methods. BN microspheres can achieve good crystallinity (>70%) and high purity (>99.9%) at 800 °C. The temperature resistance study showed that the BN microspheres could maintain the dense particle morphology at 1200 °C. In addition, the prepared BN microspheres have good dielectric and thermal conductivity properties (dielectric constant ∼3.4, dielectric loss ∼2.0 × 10−3, and thermal conductivity of 9.8 W m−1 K−1), which show great potential in the field of wave-transparent and thermal conductive composites. The synthesis process of BN microspheres proposed in this study provides a new idea for the preparation of ceramic micro or nano materials by polymer-derived ceramics. [ABSTRACT FROM AUTHOR]

Given the superior thermal stability and microwave transparency features, continuous BN fibers have displayed great potential to fulfill the increasing demand for high‐temperature structural and functional materials. The performance of BN ceramic fiber depends heavily upon its composition and microstructure. Herein, nearly stoichiometric BN fibers with high crystallinity were achieved through a facile boron trichloride (BCl3) assisted curing process. The fibers achieved a boron/nitrogen element ratio of 0.996:1 with increasing the degree of curing. After subsequent heat treatment, the fibers had uniform elemental distribution and h‐BN crystal structure. Compared with pure NH3 curing, the BN fibers cured with BCl3 demonstrated higher crystallinity (92.35%) and consistent orientation (79.8%). The existing state changes of elements revealed BCl3 assisted curing process could evenly disperse B atoms into the precursor fibers to fill the vacancies caused by nitrogen enrichment. These insights improve our understanding of preceramic fibers treatment for exploiting high‐performance BN ceramic fibers. [ABSTRACT FROM AUTHOR]

Synthesis of spinnable precursors with high ceramic yield is a crucial issue in BN fibers manufacturing by polymer‐derived ceramics (PDCs) route. Spinnability and high ceramic yield are the basis of precursor processability, formability, and mechanical properties. Herein, based on DFT simulation, we designed and synthesized a novel polymer precursor poly[2‐propylamino‐4,6‐bis(methylamino) borazine‐co‐tri(methylamino) borazine] (PPMAB) via a facile one‐pot copolymerization strategy. The random copolymerization model of condensations was obtained through DFT calculation. The molecular chain structure, molecular weight, and molecular weight distribution were optimized by adjusting the synthesis conditions. The precursor PPMAB‐1:1 with a high ceramic yield of 62.43% and concentrated molecular weight distribution (PDI < 1.3) was obtained. Rheological performance research revealed the excellent and stable spinnability of PPMAB‐1:1 by the tunable viscosity and low flow activation energy (23.61 kJ/mol). As a demonstration of spinnability, continuous BN fiber with a fine diameter (>10 km, 7.60 ± 1.13 μm, and 1.6 GPa) was obtained after melt spinning. The present work effectively improved the ceramic yield and spinnability of polyborazine precursors. Simultaneously, it can be used extensively as a universal method to prepare multifunctional copolymers with precise structure control. [ABSTRACT FROM AUTHOR]

Boron nitride (BN) interphase plays a crucial role in silicon carbide fiber-reinforced silicon carbide (SiC f /SiC) composites, because it directly influences the mechanical properties and high-temperature resistance of SiC f /SiC composites. However, fabricating a high-quality BN interphase on SiC fabrics with low cost and high efficiency remains a significant challenge. In this study, a uniform and dense BN interphase was overally prepared on 2.5D-SiC fabric by precursor-derived method. Whether it is the surface fiber or the inner fiber of the 2.5D-SiC fabric, the interphase thickness remains consistent at 550 ± 30 nm. Moreover, the obtained BN exhibits a lower crystallization temperature (approximately 1200 °C). Additionally, it maintains a strong bond with the fabric after high-temperature treatment at 1600 °C in an argon atmosphere and also retains robust adhesion after high-temperature vacuum treatment at 1600 °C. This study not only presents the overall fabrication of a uniform BN interphase on SiC fabric but also provides a novel strategy for preparing interphases on various types of fabrics. • A dense and uniform BN interphase is overall prepared by precursor-derived method for the first time. • A lower crystallization temperature of the BN precursor (∼1200 °C) is exhibited. • The BN interphase thickness of the inner and surface fibers of the fabric is 550 ± 30 nm. • At 1600 oC in Ar atmosphere, the BN interphase effectively inhibits SiC grain growth. [ABSTRACT FROM AUTHOR]

Understanding the elemental and structural evolution from green fibers to inorganic fibers is important for fabricating high-performance ceramic fibers via the polymer-derived ceramics (PDCs) route. In a previous study, we found a novel polyborazine precursor, poly[2-propylamino-4,6-bis(methylamino)borazine- co -tri(methylamino)borazine] (PPMAB), which possesses a high ceramic yield and outstanding spinnability. Herein, we provide an insight into the evolution process from PPMAB to inorganic BN fibers. During the curing process, NH 3 only reacted with the B–NHCH 3 groups in PPMAB to form B–NH 2 groups, which further reacted with B–NHCH 3 to form B–NH–B bridges. Therefore, crosslinked networks were formed in the cured fibers and the gel content increased from 52.3 to 92.1 wt%. In the nitridation process, NH 3 not only reacted with unreacted B–NHCH 3 but also reacted with B–NHCH 2 CH 2 CH 3 groups to form B–NH 2 groups, which reacted with N–CH 3 bridges in the adjacent PPMAB molecule to form –N(B 3 N 3) 3 - networks. Simultaneously, CH 3 NH 2 and CH 3 CH 2 CH 2 NH 2 escaped from the fibers, and the carbon content of the fibers was reduced from 20.588 wt% to 0.115 wt%. By controlling the curing and nitridation processes, the curing degree can be enhanced, whereas the pore defects resulting from gas molecular escape can be effectively reduced. Thus, dense inorganic BN fibers with excellent dielectric properties (2.33 < ε < 2.5, tan δ < 3.4ⅹ10−3) and high tensile strength (1.6 GPa) were fabricated. The present study not only provides ideas for the design and synthesis of polyborazine precursors, but also for the fabrication of high-performance BN fibers. [ABSTRACT FROM AUTHOR]

Continuous SiC fibers with excellent mechanical properties and high‐temperature resistance possess immense potential as reinforcements for aeroengines. In this work, we prepared functionalized boron nitride nanosheet (F‐BNNS) composite SiC (F‐BNNS/SiC) fibers by polymer‐derived method. Due to the existence of F‐BNNS, the F‐BNNS/SiC fibers demonstrate an improved tensile strength up to 2300 MPa, representing a 25% increase compared to the original SiC fibers. Furthermore, owing to the chemical bonds established between the F‐BNNS and the SiCxOy network, the decomposition of the SiCxOy amorphous phase and the crystallization of SiC at high temperatures is effectively suppressed. The F‐BNNS/SiC fibers exhibit improved high‐temperature resistance, maintaining a tensile strength of 720 MPa up to 1500°C, while the SiC fibers completely degraded under the same conditions. The successful preparation of BNNS/SiC fibers may offer new insights into the performance enhancement of advanced ceramic fibers. [ABSTRACT FROM AUTHOR]

The continuous boron nitride (BN) fiber exhibits exceptional properties such as high-temperature resistance, excellent wave transparency, and resistance to ablation. However, ceramic BN fiber fabricated from polymer-derived ceramics (PDCs) routes often suffers from low strength, which is closely related to its orientation degree and crystallization. Utilizing stretching can improve the crystallization orientation of fiber during the heat treatment process, we present a novel approach to address this issue in this study. Graphite rings are employed to provide continuous micro-tension by restricting the shrinkage of BN fiber during the polymer-to-ceramic conversion. The micro-tension level is adjusted by the diameter variation of the graphite rings. A series of analyses reveal a significant enhancement in the orientation degree of the ceramic BN fiber, transforming it from disordered to highly oriented (with ω value of 0.84). The orientation factor (f) grows by nearly 20%. Furthermore, the crystallization increases from 81.13% to 90.72%. The ratio of pore area to cross-section area of ceramic BN fibers decline from 0.63% to 0.19%. Correspondingly, the bulk density is increased by nearly 44%. Consequently, the strength and Young's modulus of the fiber are enhanced by approximately 3 and 10 times, respectively. [ABSTRACT FROM AUTHOR]

Ceramic temperature sensors are promising for automobile and jet engine monitoring but lack suitable materials and manufacturing methods. Herein, we designed and synthesized a novel vinyl-bridged graphene oxide/polycarbosilane (v-GO/PCS) hybrid precursor that could be easily shaped and pyrolyzed into reduced graphene oxide/silicon carbide (rGO/SiC) film as a stable and sensitive temperature sensor under harsh environments. As a result, 1 %-rGO/SiC ceramic film exhibited a fast response in the temperature range of 100 °C to 1000 °C. The rGO acted as a bridge linking the surrounding SiC grains, which significantly reduced the amount of thermal energy required for the directional transfer of electrons, leading to an increase in both the detection range and sensitivity of the sensor. The introduction of dry air, water vapor and acid (or alkali) atmosphere in the high-temperature conditions further verified its excellent stability. This work provides a simple and practicable strategy for highly stable temperature detection under harsh environments. [ABSTRACT FROM AUTHOR]

• A self-supporting and hierarchical electrocatalyst of Ni@NCNTs-CNF is developed. • Ni@NCNTs-CNF has dual active sites for CO2 reduction and H2 evolution, respectively. • Certain syngas is produced with ∼100 % FE and j = 25 mA/cm2 at low potential of -0.8 V. • The active sites-dependent syngas proportion enables a wide CO/H2 ratio (1:4∼5:1). Electroreduction of aqueous CO 2 is a sustainable approach to produce tunable syngas, which demands the rational design of dual-active-sites catalysts for individual CO 2 reduction reaction (CRR) and hydrogen evolution reaction (HER). Here, the self-supporting nickel@nitrogen-doped carbon nanotubes grown on carbon nanofiber (Ni@NCNTs-CNF) is developed with Ni single atoms (SAs) and nanoparticles (NPs) functioning as CRR and HER sites respectively. Benefiting from the hierarchical nanostructures for effective electron transport and mass diffusion, as well as the high accessibility of dual-active sites, Ni@NCNTs-CNF catalysts can mediate aqueous CO 2 into CO/H 2 (2:1) with a 100 % Faraday efficiency and a high current density (25 mA cm−2) at low working potential (-0.8 V). By manipulating the Ni SAs/Ni NPs ratio, the CO/H 2 proportion is easily modulated in a wide range from 1:4 to 5:1, independence of the working potential (-0.6 to -0.8 V). Our findings provide fresh insights into the design of dual-active-sites catalysts. [ABSTRACT FROM AUTHOR]